A pacemaker is a medical device, typically implanted within a patient, which recognizes various dysrhythmias such as an abnormally slow heart rate (bradycardia) or an abnormally fast heart rate (tachycardia) and delivers electrical pacing pulses to the heart in an effort to remedy the dysrhythmias. An ICD is a device, also implantable into a patient, which additionally recognizes atrial fibrillation (AF) or ventricular fibrillation (VF) and delivers electrical shocks to terminate fibrillation.
Pacemakers and ICD's carefully monitor characteristics of the heart such as the heart rate to detect dysrhythmias, discriminate among different types of dysrhythmias, identify appropriate therapy, and determine when to administer the therapy. The heart rate, for example, is monitored by examining the electrical signals that are manifest concurrent with the depolarization or contraction of the myocardial tissue of the heart. The electrical signals are detected internally by sensing leads mounted within the heart and are referred to as internal electrocardiogram (“IEGM”) signals. The normal contraction of atrial muscle tissue appears as a P-wave within the IEGM. A sequence of consecutive P-waves defines the atrial rate. The normal contraction of ventricular muscle tissue appears as an R-wave (sometimes referred to as the “QRS complex”) within the IEGM. A sequence of consecutive R-waves defines the ventricular rate. If the heart is subject to flutter or fibrillation, P-waves and R-waves typically cannot be discerned within the IEGM. Hence, the pacemaker or ICD may need to rely on other characteristics of the IEGM to discriminate among different types of flutter and fibrillation, to identify optimal therapy, and to determine when to administer the therapy. Some state of the art pacemakers and ICDOs are capable of sensing electrical signals independently in the atria and in the ventricles. Hence, an atrial IEGM and a separate ventricular IEGM are detected. The atrial rate is determined based upon P-waves detected in the atrial IEGM. The ventricular rate is determined based upon R-waves detected within the ventricular IEGM.
Thus pacemakers and ICD's administer therapy to the heart, in part, based upon the detection of electrical characteristics of the heart such as P-waves, R-waves, atrial rate, ventricular rate, and the like. As one specific example, if the atrial and ventricular rates are both below a minimum acceptable heart rate threshold or if long gaps appear within the IEGM signals wherein no P-waves and R-waves are sensed, the cardiac pacing device thereby concludes that the patient is suffering from bradycardia and administers pacing pulses in an effort to increase the heart rate or to eliminate long gaps without heart beats. As another specific example, if the atrial and ventricular rates are well above a maximum expected heart rate, the cardiac pacing device concludes that the patient is suffering from a tachyarrhythmia and administers appropriate therapy such as, for example, overdrive pacing in an effort to lower the heart rate to within an acceptable range. If the atrial rate is found to be extremely high, but the ventricular rate is relatively normal, the cardiac pacing device concludes that the patient is suffering from atrial flutter or atrial fibrillation and administers a defibrillation pulse to the atria. If the ventricular rate is extremely fast and chaotic, the cardiac pacing device concludes that the patient is suffering from ventricular fibrillation and administers a defibrillation pulse directly to the ventricles. Details regarding techniques for discriminating between atrial and ventricular dysrhythmias or arrhythmias are provided in U.S. Pat. No. 5,620,471 to Duncan entitled “System and Method for Discriminating Between Atrial and Ventricular Arrhythmias and for Applying Cardiac Therapy Therefor”, issued Apr. 15, 1997, which is incorporated by reference herein.
Reliable operation of pacemakers and ICD□s therefore necessitates that the device be capable of accurately detecting P-waves, R-waves or other electrical events originating within the heart. Insofar as P-waves are concerned, however, the aforementioned R-waves, though initially generated within the ventricles, propagate into the atria and may be detected therein as part of the atrial IEGM signal. It is therefore possible for the device, upon detecting an electrical pulse within the atria, to misidentify a far-field R-wave as being a P-wave. As a result, any functions performed by the pacemaker which require accurate determination of P-waves may not function as intended. For example, PVCs may be classified as P—R events so that the calculated atrial rate will be higher than the actual atrial rate, perhaps causing the device to erroneously conclude that the atria are subject to a tachyarrhythmia, which does not in fact exist, or classify a ventricular tachycardia as an atrial tachycardia. Alternatively, the overestimated atrial heart rate may cause the device to fail to detect a bradycardia, which does exist. As a result, inappropriate therapy may be administered. For an ICD, an erroneously high determination of the atrial rate may cause the ICD to incorrectly conclude that the heart is subject to atrial fibrillation, resulting in a potentially painful cardioversion pulse administered to the atrium.
Thus, it is necessary to properly distinguish P-waves or other electrical events originating in the atria from far-field R-waves or other events originating in the ventricles. Accordingly, most state-of-the-art pacemakers ignore any events detected within the atria during a predetermined period of time subsequent to the detection of an R-wave in the ventricles. This period of time is referred to as the post-ventricular atrial blanking (PVAB) interval or a post-ventricular atrial refractory period (PVARP). Briefly, upon the detection of an R-wave from a sensing electrode positioned within the ventricles, the pacemaker thereafter ignores any events detected from a sensing lead within the atria for a period of time (e.g. 225 ms.) under the assumption that any event detected during that period of time is actually a far-field R-wave.
The use of the PVAB interval presupposes that the R-wave will be detected in the ventricles before it appears as a far-field R-wave in the atria. This is not always the case. The inventors of the present invention have determined that circumstances can arise wherein a far-field R-wave is detected within the atria before it is detected within the ventricles. This may occur, for example, if an atrial sensing lead is positioned closer to the source of an R-wave than the ventricular sensing leads. Another circumstance wherein an R-wave may be detected within the atria without a preceding R-wave detection in the ventricles occurs if the threshold for R-wave detection in the ventricles is set too high, such that some R-waves are not detected at all within the ventricles. In any event, if the far-field R-wave is detected within the atria without an immediately preceding R-wave detection in the ventricles, the aforementioned PVAB interval is ineffective to filter out the far-field R-wave from the atrial IEGM. As a result, far-field R-waves are misclassified as P-waves resulting in incorrect determination of atrial rate, or other critical parameters, causing potentially erroneous therapy to be administered by the pacemaker. Although these problems have been described primarily with reference to the discrimination of P-waves from far-field R-waves, similar problems arise even in circumstances wherein P-waves and R-waves cannot be discerned within the IEGM, such as during flutter or fibrillation.
Accordingly, it would be highly desirable to provide an improved technique for discriminating P-waves or other electrical events originating within the atria from far-field R-waves or other electrical events originating within the ventricles, and it is to that end that aspects of the present invention are primarily directed.